Spread spectrum communications involves spreading the transmitted signal energy over a wide bandwidth utilizing a spreading function which is known at both the transmitter and receiver. The spreading function is typically defined by a binary sequence which is usually pseudo randomly generated. These sequences are often referred to as the chip sequences, which include binary symbols referred to as chips.
A common spread spectrum technique is known as direct sequence spreading. Direct sequence spreading includes directly multiplying a conventional narrow band signal by the chip sequence, where the chip rate is higher than the data rate.
Pulse shaping techniques are utilized in spread spectrum systems in order to improve the modular spectral efficiency and minimize adjacent channel interference. Quasi band-limited minimum shift keying (QBL-MSK) is a pulse shaping technique which provides a significant improvement in modulator spectral efficiency over standard minimum shift keying (MSK) techniques, when operated with a linear power amplifier.
QBL-MSK pulse shaping is applied to spread spectrum radios by utilizing a binary phase shift keying (BPSK) technique to convey the data information. By utilizing BPSK data modulation, the QBL-MSK waveform structure is unmodified. This enables the advantages provided by the QBL-MSK pulse shaping to be directly applied to the spread spectrum signal.
A technique known as quadrature phase shift keying (QPSK) is utilized to increase the data capability of a spread spectrum signal generated by QBL-MSK modulation. Utilizing the QPSK technique results in the data rate being increased by a factor of two.
U.S. application Ser. No. 11/314,757, filed on Dec. 21, 2005 by the same inventor of the present application, discloses a serial quasi-band-limited minimum shift keying (SQBL-MSK) modulation waveform. The SQBL-MSK waveform enables a serial correlation structure to be used, with an in-phase (I) and quadrature (Q) correlator using the same synchronization (SYNC) sequence. Standard parallel QBL-MSK uses a four correlator structure based on the even and odd symbols on the I and Q sequence. The serial correlation structure used for synchronization (SYNC) detection provides a simplified binary phase shift keying (BPSK) correlation operation versus a parallel structure. This reduces the complexity of the SYNC operation.
To simplify the SYNC operation, serial formatting is applied to quadrature pulse-shaped signals. This is implemented by adding a serial formatting term to the modulation waveform, which multiplies a nonreturn to zero (NRZ) symbol sequence with a repetitive 1, 1, −1, −1 sequence. For SQBL-MSK, the modulation waveform for the SYNC portion of the waveform may be written as follows:
      s    ⁡          (      t      )        =                    [                              ∑                          i              =              0                                      M              -              1                                ⁢                                          ⁢                                                    (                                  -                  1                                )                            i                        ⁢                                          c                21                            ·                              p                ⁡                                  (                                      t                    -                                          2                      ⁢                                                                                          ⁢                                              iT                        s                                                                              )                                                                    ]            ⁢      cos      ⁢                          ⁢              (                  2          ⁢                                          ⁢          π          ⁢                                          ⁢                      f            o                    ⁢          t                )              +                  [                              ∑                          i              =              0                                      M              -              1                                ⁢                                          ⁢                                                    (                                  -                  1                                )                            i                        ⁢                                          c                                                      2                    ⁢                                                                                  ⁢                    i                                    +                  1                                            ·                              p                ⁡                                  (                                      t                    -                                                                  [                                                                              2                            ⁢                                                                                                                  ⁢                            i                                                    +                          1                                                ]                                            ⁢                                              T                        s                                                                              )                                                                    ]            ⁢      sin      ⁢                          ⁢              (                  2          ⁢                                          ⁢          π          ⁢                                          ⁢                      f            o                    ⁢          t                )                            and        
      p    ⁡          (      t      )        =      {                                                                      [                                                      sin                    ⁡                                          (                                                                        π                          ⁢                                                                                                          ⁢                          t                                                                          2                          ⁢                                                                                                          ⁢                                                      T                            s                                                                                              )                                                                            (                                                                  π                        ⁢                                                                                                  ⁢                        t                                                                    2                        ⁢                                                                                                  ⁢                                                  T                          s                                                                                      )                                                  ]                            3                        ;                                                                              -                2                            ⁢                                                          ⁢                              T                s                                      ≤            t            ≤                          2              ⁢                                                          ⁢                              T                s                                                                                      0            ;                                                elsewhere.                              
For the data modulated SQBL-MSK waveform equation given above, Ts represents the symbol period, ci represents the SYNC symbols at time iTs, 2M is the number of symbols in the SYNC sequence, p(t) is the QBL pulse-shaping function, fo, is the carrier center frequency, and the (−1)i terms which multiplies the symbol value represents the serial formatting. For 128 symbol SYNC, for example, M is equal to 64. The SYNC symbols (ci) take on either a +1 or −1 value.
During communications between one node (receiver and transmitter system) and another node, data packets may be transmitted and received. These data packets may include various messages used for different purposes. For example, the data packet may include a power amplifier (PA) ramp up message, an automatic gain control (AGC) acquisition message, multipath (MP) window message, a SYNC message, a time of arrival (TOA) message, a data message and a (PA) ramp down message. The TOA message may be used by a quadrature multiple frequency ranging (QMFR) module of a receiver to separate multiple path signal components between a transmitter of one node and a receiver of another node.
A QMFR module, or QMFR processor, that receives the TOA message and executes an algorithm to separate multiple path signal components requires that the sliding correlation output for zero phase error should be only on the I correlation output and not on the Q correlation output. This requirement cannot be met when using SQBL-MSK modulation.
Furthermore, conventional QBL-MSK modulation of a signal provides both an I and Q correlation output. The isolation of the correlation output to either the I or Q side is required to implement the QMFR algorithm. The QBL-MSK modulated signal must, therefore, be reformatted in order to implement the QMFR algorithm.
The present invention addresses, among other things, an advantageous communication system and method of (1) using QBL-MSK modulation for implementing the QMFR algorithm during the TOA message portion of the data packet, and (2) using SQBL-MSK modulation for processing other portions (for example SYNC message or data message) of the data packet.